The present invention relates to a color cathode ray tube having an improved screen and a manufacturing method thereof, and more particularly to a color cathode ray tube and a manufacturing method thereof which effectively prevents a metal back, an element of the screen, from desquamating.
FIG. 1 is a sectional view of the conventional CRT. Generally, as shown in FIG. 1, a color cathode ray tube comprises a panel 10 on whose inner wall a screen 11 is formed so as to have a shadow mask 50 installed in front of screen 11, a funnel 20 bonded with panel 10 to form a vacuum bulb, an electron gun 30 installed inside a neck 21 provided at the rear end of the funnel, and a deflection yoke 40 installed on the outside of neck 21. In order to reproduce an image, electron beams emitted from the electron gun are deflected to land on precise locations on the phosphor screen by the deflection yoke, thus forming a pixel, a plurality of which complete a picture. Here, optimal pictures need the elements constituting a CRT to be in their best condition. Particularly, the formation of the phosphor screen on which an image is formed is a critical factor in obtaining a good-quality picture.
FIGS. 2 and 3 are enlarged views of a portion A of the conventional CRT of FIG. 1. Referring to FIG. 2, screen 11 formed on the inner surface of panel 10 is composed of a black matrix 12, a phosphor layer 13, a film 16 and a metal back 14. Also, black matrix 12 and phosphor layer 13 are formed directly on the surface of panel 10, film 16 covers the whole surface of black matrix 12 and phosphor layer 13, and metal back 14 covers film 16.
FIG. 3 shows a sectional view of the conventional CRT after heat-processing the panel. During heat-processing the panel, film 16 is vaporized and decomposed and escapes through metal back 14. Film 16 is an organic decomposable thin film and is comprised of a synthetic resin containing acryl emulsion. As shown in FIG.3, narrow gaps 10' are formed in the space between metal back 14 and the surface of black matrix 12 and phosphor layer 13, the space having been occupied by film 16. Also, metal back 14, which has a thickness of nearly ten microns, becomes partially rugged and makes contact with black matrix 12 and phosphor layer 13. Here, the metal back layer is for reflecting all of the light generated in the screen 11 to the panel 10 of the CRT.
FIG. 4 shows a process of manufacturing a conventional screen having the above structure, which will be described below.
(a) Black matrix forming step: A liquid graphite solution containing positive photoresist is coated over the entire panel and then exposed to ultraviolet light through the shadow mask holes, to harden in precise locations in the form of dots or lines. After the exposure, the unhardened portion of the photoresist film is washed away, and then the remainder is dried. The graphite which remains is in contact with the panel and has a striped pattern or dotted pattern, that is, a black matrix.
(b) Phosphor layer forming step: Red, green and blue phosphors are formed on the bottom of the panel, for example, in a striped pattern or a dot-trio pattern. In this step, R, G and B phosphor slurry is used and ordinary photolithography is used, which comprises an exposing step using a pattern mask and a developing step in which the exposed portion is selectively etched.
(c) Film forming step: A filming liquid of a synthetic resin containing acryl emulsion in a predetermined thickness coats the surface of the phosphor layer and the black matrix formed in a predetermined pattern, and is dried to form a film 16. The film 16 planarizes the rough phosphor layer 13, and prevents the penetration of aluminum into the phosphor screen in a subsequent process. This functions to even the surface of the phosphor layer 13 having non-uniform particles and to bury exposed cavities of the phosphor layer 13 so as to improve the planarization of a metal back 14 produced in a later-mentioned step. This step includes applying the filming liquid to the panel and drying it so as to have a specific hardness.
(d) Metal film forming step: A metal film is formed on the overall phosphor layer 13 in which the film 16 is formed. The metal film is generally made of aluminum, and called a metal back 14. The metal back 14 is formed by an evaporation method using a vacuum chamber.
(e) Baking step: After the above steps, the panel is heated to pyrolyze and eliminate the film 16 interposed between the phosphor layer and the metal film back 14. Here, the heating temperature is about 400.degree. C.
Through the above steps, the screen is completed on the inner surface of the panel. Since the film 16 is eliminated by pyrolysis, gaps are produced under the metal back 14. The metal back 14 is partially in contact with the phosphor layer 13 and black matrix 12 and the whole exposed surface is uniform.
The conventional manufacturing process has the following problems.
Since the film 16 has to coat the whole surface of the black matrix and phosphor layer 13 including the panel periphery and skirt, using a slurry coating or spray coating method while the glass panel 10 and top of the graphite 12 are very slippery, the skirt and periphery have a greater tendency for desquamation or blistering. The graphite particles are much smaller than the phosphor particles used for the screen. The film 16 becomes swollen or desquamated during the bake-out process after the formation of the metal back 14, since a great deal of gas is locally generated due to pyrolysis. Such desquamation takes place frequently at the corners of the panel around the screen where a great deal of gas is generated, as well as in the image display portion of the screen. The desquamation also occurs frequently due to vibration during a dust cleaning process after the completion of the screen and before the completion of the CRT.
The desquamation of the metal back 14 may happen even in the finished product because the metal back 14 is weakly adhered to the black matrix 12 and the phosphor layer 13. That is, since the film 16 under the overall surface of the metal back 14 is substantially separated from the phosphor layer 13 and the black 14 matrix 12, the metal back is subject to desquamation even due to slight vibrations or impacts. As a result, the problem deteriorates the quality of the whole screen including the phosphor layer 13 and impedes realization of a good-quality picture. Further, the pieces of the desquamated metal back 14 stick to the shadow mask, closing the beam passing holes and rendering the product useless.